Size is an important parameter in nanoscale materials that can provide control over many of their physical and chemical properties, including luminescence, conductivity, and catalytic activity. Over the past century, colloid chemists have gained excellent control over particle size for several spherical metal and semiconductor compositions. This chemical control over particle size has led to the discovery of quantum confinement in colloidal nanocrystals and their exploitation as probes in biological diagnostic applications, LED materials, lasers, and Raman spectroscopy enhancing materials. In contrast, the challenge of synthetically controlling particle shape has been met with limited success. Nevertheless, some physical and solid-state chemical deposition methods have been developed for making semiconductor and metal nanowires, nanobelts, and dots, and there are now a variety of methods for making rods with somewhat controllable aspect ratios using electrochemical and membrane-templated syntheses.
Less is known with respect to solution synthetic methods for non-spherical particles such as triangles or cubes. However, methods do exist for making colloidal samples of Pt cubes and pyramids (Ahmandi et al., Science 272:1924 (1996)), and PbSe, CdS, and Ni triangles (Fendler et al., J. Am. Chem Soc. 122:4631 (2000), Pinna et al., Ad. Mater. 13:261 (2001), Klasu et al., Proc. Natl. Acad. Sci U.S.A. 96:13611 (1999)). Promising recent work has resulted in methods for synthesizing BaCrO4, CdSe and Co nanorods and distributions of arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals (Li et al., Nature 402:393 (1999), Peng et al., Nature 404:59 (2000), Puntes et al., Science 291:2115 (2001), Manna et al., J. Am. Chem. Soc. 122:12700 (2000)). All of these solution methods are based on thermal processes, and in most cases, with the exception of rods, yield relatively small quantities of the desired particle shape. However, much like particle size-control in nanoscale materials led to the discovery of new and important fundamental science and technological applications in diagnostics, optics, catalysis, and electronics, synthetic methods that lead to control over particle shape can be expected to lead to important fundamental as well as technological advances. Therefore, the development of bulk solution synthetic methods that offer control over particle shape is of paramount importance if the full potential of these novel materials is to be realized.